Active and durable R2MnRuO7 pyrochlores with low Ru content for acidic oxygen evolution

The production of green hydrogen in water electrolyzers is limited by the oxygen evolution reaction (OER). State-of-the-art electrocatalysts are based on Ir. Ru electrocatalysts are a suitable alternative provided their performance is improved. Here we show that low-Ru-content pyrochlores (R2MnRuO7, R = Y, Tb and Dy) display high activity and durability for the OER in acidic media. Y2MnRuO7 is the most stable catalyst, displaying 1.5 V at 10 mA cm−2 for 40 h, or 5000 cycles up to 1.7 V. Computational and experimental results show that the high performance is owed to Ru sites embedded in RuMnOx surface layers. A water electrolyser with Y2MnRuO7 (with only 0.2 mgRu cm−2) reaches 1 A cm−2 at 1.75 V, remaining stable at 200 mA cm−2 for more than 24 h. These results encourage further investigation on Ru catalysts in which a partial replacement of Ru by inexpensive cations can enhance the OER performance.

3 Table S1. Unit-cell parameters, atomic positions, occupancies, thermal factors and reliability factors of R2MnRuO7 in the cubic Fd-3m (no. 227) space group.

S1.1.4 Electrochemical Active Surface Area
The electrochemical active surface area (ECSA) was based on the double-layer capacitance (Cdl) of the surface of the pyrochlores. Note that we prepare the regular inks but without adding vulcan. Cyclic voltammograms around the open circuit potential (OCP) in Ar were performed where the only processes occurring are supposed to be due to the double-layer charging. The cyclic voltammograms are carried out at 2, 5, 10, 25 and 50 mV s -1 and the double-layer charging current (ic) is equal to the product of the scan rate (ν) and Cdl at a constant potential.
Plotting ic vs. ν gives Cdl as the slope. Then ECSA is equal to Cdl divided by the specific capacitance (CS) of an atomically flat planar surface of the compounds per unit area under the same electrolyte conditions. A typical value for CS of 0.035 mF/cm 2 was used as it was previously used for several oxides in the same electrolyte. [2][3][4] The ECSA values are shown in Table S3.

S1.1.5 Assessment of the evolution of Cdl and ECSA with OER cycling
The experiments were conducted with Y2MnRuO7. An ink containing 0.05 mgY2MnRuO7, 0.01 mgvulcan, 0.0003 mLNafion and 0.0097 mLTHF was dropped onto the working electrode to a final Ru loading of 0.011 mg. EIS experiments were recorded at open circuit potential after cycling.
ECSA is directly related to Cdl, and can be determined by CV or EIS. Both techniques provide similar information on the capacitive behaviour of the system. However, by measuring the capacitance behaviour by EIS, the Cdl can be replaced by a CPE and thus more information can be obtained, such as the evolution of the (non)-ideal capacitance of the surface. CPE can be expressed as: = 1 ( ) (Eq. 1) where Tdl is a double-layer capacitance parameter, ω is the angular frequency of the ac perturbation and φ is the dimensionless CPE exponent representing the ideality or non-ideality of a capacitance. If φ = 1, the system acts as an ideal capacitance and as it decreases, so does the ideality.
The change on Tdl and φ with the OER cycle is given in Figure S3. Three main regions can be observed. In the first one, Tdl varies towards positive values due to the restructuring of the catalyst. In that same region, φ deviates to lower values indicating that the surface is less regular and, therefore, a less ideal capacity is implied. In the second region, the values of Tdl and φ remain constant. Finally, in the region of deactivation, the Tdl values drop sharply until the complete deactivation of the catalyst. 6 In the case of a redox reaction from the CPE, a value of Cdl can be obtained by the following equation: The initial ECSA value was 9.3 cm 2 , at the stable zone ECSA was around 12 cm 2 and the final value was 3.1 cm 2 . Figure S3. Evolution of the double-layer capacitance and  with OER cycles. Three regions can be observed, surface reconstruction (blue), stable surface (green) and degradation (orange) in which the double-layer capacitance increases, remains stable and decreases, respectively.

S1.1.6 Mass-specific surface area (AS) calculation
Mass-specific surface areas (AS) are calculated using TEM data assuming that the particles are close to a spherical geometry. 5,6 AS was determined using the following formula: where d are the diameters of each particle calculated by TEM, dv/a is the volume/area diameter (dv/a= Ʃd 3 /Ʃd 2 ), ρ is the pyrochlore bulk density (ρ= Mw · Z / NA· Vf.u., where Mw is the molecular weight, Z is the number of formula units per cell, NA is Avogadro's number, and Vf.u. is the volume of the formula unit).  Ru and Mn 3p lines are also doublet states (3p1/2 and 3p3/2), however, for this study only the 3p3/2 are used ( Figure S5). Ru 4+ and Mn 4+ are the main oxidation states in the fresh sample. 7,[11][12][13] We did not observe a significant variation of the oxidation state of these cations during the OER, or a change in the shape or in the contributions observed for both cations. A shake-up satellite peak was considered in the fitting due to core-hole screening and/or the asymmetry of the main peak. 7,11 No significant variations in this satellite component were observed during the electrochemical characterization.

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)
The dissolution of cations from Y2MnRuO6 during OER cycling, at 100 and 6000 CV cycles, were determined by averaging two different experiments. The electrolytes were collected after both experiments and analysed by ICP-OES to determine the concentration of cations dissolved. Note that in the CV measurements we used 100 mL of electrolyte and 0.05 mg of pyrochlore. Therefore, the maximum concentration of cations that can be dissolved is 0.1994 mg L -1 of Y, 0.0661 mg L -1 of Mn and, 0.1134 mg L -1 of Ru.

S1.2. Supplementary computational details
The surface structure of the dissolved Y2MnRuO7 pyrochlore was built consecutively by the following procedure: I) We simulated the bulk structure of Y2RuMnO7. II) We calculated the most stable surface facet of Y2RuMnO7 by calculating the surface energies for different surface terminations. We found that the (111) facet was the most stable, with a surface energy of 0.04 eV Å -2 , compared to the (100) facet with 0.06 eV Å -2 and the (110) facet with 0.14 eV Å -2 . III) We evaluated the OER performance of the sites on this surface and found onset potentials surpassing 2.00 V, which indicates that a pristine, stoichiometric slab is not representative of the surface present under experimental conditions. 11 IV) Since it is known that oxide surfaces with non-noble metals such as that of Y2MnRuO7 tend to leach metal atoms into the solution under acidic electrochemical conditions, 15,16 we then removed the Y atoms from the topmost surface layers. V) From the structure described in the previous step, we calculated the adsorption energies of *O, *OH and *OOH on the Ru sites. The loss of Mn was calculated by replacing Mn by Ru in the topmost layers and re-relaxing. Figure S7 shows top views of the converged geometries of the clean surface and the surface with *O, *OH and *OOH. Figure S8 shows the corresponding side views. Note that lattice oxygen atoms and oxygen atoms in the adsorbates are depicted in different colors. Figure S9 shows the varying amounts of Mn and Ru in the topmost layers of the slabs.    Table S7. The method to add the experimental datapoint of Y2MnRuO6 in Figure 6 is based on ref. 17 The method uses the experimental OER overpotential corresponding to a current density of 1 mA cm -2 As, which for Y2MnRuO6 is around 0.29 V, according to Figure S2c. That overpotential is used to approximate ∆ − ∆ by means of the semiempirical volcano plot in ref. 17 In particular, 0.29 V corresponds to ∆ − ∆ ≈ 1.53 , knowing that Y2MnRuO7 is on the strong-binding side of the volcano, as revealed by our DFT calculations (see Figure 6 in the main text).